1. Field of the Invention
The invention relates to method of controlling a metallic vapour discharge lamp, which is used as an ultraviolet light source.
2. Background of the Disclosure
It is generally known in technical fields, such as, photochemical reactions, and the curing of paints and inks or the like, to use ultraviolet rays in a wavelength range of 250 to 400 nm. The term "paint" is also intended to cover the more general term "lacquer".
Normally a metallic vapour discharge lamp is used as such an ultraviolet radiation source. For a more optimized use with respect to the above-described purposes, use is made of a metallic vapour discharge lamp in which is encapsulated iron, which has a plurality of line spectra in a wavelength range of 350 to 400 nm.
However, if such a metallic vapour discharge lamp remains in operation for a long time, due to the adhesion of the iron to the inner wall of the arc tube a thin film is formed. The problem then arises that the quantity of iron contributing to the luminescing action decreases, and simultaneously the thin film prevents the passage of the ultraviolet rays. Consequently the intensity of the ultraviolet rays is significantly reduced.
In order to solve the above problem a measure is performed, in which e.g. at least one of the metals lead, tin, thallium, cadmium, magnesium, bismuth or the like is selected or metal halide is added and in which simultaneously the arc tube temperature is regulated to 550.degree. to 800.degree. C. As is known, this measure prevents the formation of the thin iron film over a long period of time.
However, if the arc tube temperature is below 550.degree. C., the encapsulated metal halide condenses in a low temperature part within the arc tube and as a result a desired emission cannot be obtained. Also, if the are tube temperature rises above 800.degree. C., it is not possible to adequately perform a halogen cycle within the arc tube, which leads to an adhesion of a thin film of the metals, particularly iron, within the are tube. It is therefore necessary to keep the arc tube temperature constant in a range 550.degree. to 800.degree. C.
Such a metallic vapour discharge lamp has been conventionally used in such a way that it is operated with an input power of equal to or lower than 160 W/cm lighting length and is simultaneously cooled by the exhausting or blowing out of air, as shown in FIG. 2.
However, of late, there has been a need for a stronger intensity or output of the ultraviolet rays. In order to cover this requirement it is necessary to operate the lamp with an input path of greater than 160 W/cm lighting length. However, in this case it is also necessary to increase the cooling capacity of the lamp and it is found that the arc tube temperature cannot be maintained in the range 550.degree. to 800.degree. C. using the techniques of the prior art.